For more than a century, ecologists have sought to understand the
forces determining food chain lengths in natural communities (Post and
Takimoto, 2007) and to explain why food chain lengths differ between
aquatic and terrestrial systems (Chase, 2000; Shurin et al., 2006).
During that time multiple explanations have been proposed. These include
species richness (alone or in combination with ecosystem size; Cohen and
Newman, 1991; Post et al., 2000; Takimoto et al, 2008), productivity and
nutrient availability (Oksanen et al., 1981; DeAngelis et al., 1989;
Kaunzinger and Morin, 1998), disturbance regimes (Pimm and Lawton, 1978;
Power et al, 1996; along with productivity in Townsend et al, 1998),
ecological stoichiometry (Diehl and Feissel, 2000), size-based patterns
in predator-prey interactions (Cohen et al, 1993; Jennings and Warr,
2003; Williams and Martinez, 2004), food-web structure (Long et al,
2011), and metacommunity dynamics (Calcagno et al, 2011). The references
offered here are but a small sampling of the attempts to understand the
mechanisms affecting food chains. Among the oldest explanations is the
idea animal metabolism might determine food chain length. Elton (1927)
argued that the general constraint on food chain length may be largely
related to the loss of energy in successive trophic levels due to low
conversion efficiency in consumer species. This argument extends from
the basic thermodynamic principle that energy dissipates from a system
as a consequence of inefficient transfer or conversion and has been
expanded and refined by Lindeman (1942) and Yodzis (1984), among others.

In ecological systems this is best expressed by Lindeman's
concept of ecological efficiency, which is represented as the
"ratio of the energy flux into [a] trophic level to the energy flux
into the level below" (Colinvaux and Barnett, 1979). This is itself
directly related to the conversion efficiency of consumers (Slobodkin,
1962) and leads to the observation that organisms are able to convert
only a fraction of consumed resources into their own biomass. Now termed
the "trophic efficiency hypothesis" (e.g., Jenkins et al.,
1992, or previously the "energy flow hypothesis," Hutchinson,
1959), the basic idea is that some fraction (often very small) of the
energy (usually expressed as biomass) entering any trophic level will be
fixed as biomass in the subsequent trophic level. This is thought to be
the mechanism responsible for the almost-universally observed biomass
and abundance pyramids in ecological communities (e.g., Trites, 2003).
Essentially, the metabolic explanation for constraints on food chain
lengths contends higher trophic levels are simply energy starved, and at
some point, cannot support an additional level of consumers (predators).
It is important to note such mechanisms might apply both to individual
food chains and to the trophic structure of entire communities.

Two predictions extend from the trophic efficiency hypothesis.
First, while trophic structure may be organized with respect to gape
limitation and body size distributions among consumer and resource
species (Roberts, 2003), body size should also play a role in
determining food chain lengths. For animals metabolic rate scales with
body size as a 3/4 power law (e.g., Brown et al, 2004), and therefore,
larger consumer species have substantially larger energetic requirements
(the scaling of metabolic rate to body size is different for autotrophs,
as well as microbes and protists; see Pretzsch and Dieler 2012 and
DeLong et al. 2010 respectively). This, combined with the fact that
population density decreases with increasing body size on a similar
logarithmic scale, could act to bottleneck energy passing through food
chains. For example large herbivorous species might limit energy flow up
trophic levels relative to smaller taxa that utilize the same resources,
and thereby shorten food chains. Second, metabolic costs associated with
endothermy should also influence food chain lengths. Energetic demands
are roughly an order of magnitude greater in endotherms relative to
ectotherms of the same size (e.g., Humphreys, 1979; Bennet and Ruben,
1979; Hayes, 2010; recently reviewed by Glazier, 2014). Therefore,
systems containing more endotherms should display shorter food chain
lengths. Additionally, the trophic position of endotherms should impact
food chain lengths; endothermic herbivores should result in shorter
chain lengths.

In the past three decades, many high-resolution consumer-resource
networks (as well as whole food webs) have been produced (e.g., Hall and
Raffaelli, 1991, Williams and Martinez, 2000; Brose et al., 2005;
Woodward et al., 2008), and have been used to examine, among other
things, the patterns and relationships between consumer-resource body
sizes and metabolic modes (e.g, Brose et al., 2006; Schmitz and Price,
2011). Further, patterns of variation in metabolic efficiencies between
modes (endothermy and ectothermy) have been considered in the context of
the trophic structure of ecological networks (e.g, Riede et al., 2011;
Moore and de Ruiter, 2012). The possibility of increased endotherm-based
constraints on terrestrial food chains was considered by Ryszkowski and
French (1982), and Yodzis (1984) attempted to directly explore
predictions extending from this hypothesis, using a collection of
published food webs available at that time (N = 34, all of which were
quite small and poorly resolved by today's standards). Because
attributes like body size, metabolic mode and trophic position vary
among taxa in different systems, consideration of the food webs
available today should offer a more thorough assessment of any
relationships between these variables and food chain length. Using the
recently completed GlobalWeb database (http://globalwebdb.com/),
augmented with several other large published food webs, we assess many
of the predictions that extend from the trophic efficiency hypothesis.
Specifically, we test the predictions that: (1) increasing the fraction
of endothermic taxa in a system will lead to shorter food chains. (2)
Increasing the fraction of endothermic taxa residing at lower trophic
levels (e.g., herbivores) will produce shorter average food chains. (3)
Increasing the fraction of large consumers at lower trophic levels will
shorten food chains. We predict that these effects would be expressed in
the mean food chain length (MFCL) of entire systems and therefore could
be detected by comparing MFCL across systems. Finally, an alternative
prediction consistent with the energy efficiency hypothesis is that
systems conforming to one or more of the above scenarios would also
contain fewer total food chains than expected given food-web size. To
address each prediction individually, we attempt to separate variables
related to body size, metabolic mode and trophic position, and determine
the strengths of these variables as predictors of food chain length.

METHODS

DATA AND FOOD WEB PROPERTIES

Our data were largely drawn from the recently constructed GlobalWeb
database (Thompson et al., 2012), which is now the largest single
collection of published food webs in the world. Thompson and colleagues
discuss in some detail the relationship between food web size (which is
tightly correlated with species richness) and resolution. In general
smaller food webs produce fewer food chains and shorter MFCLs that
larger ones. However, our purpose was not to compare large and small
food webs but to compare food webs of equal sizes across ecosystem
types. That is, we predicted that differences in MFCL for systems of
equal size would correlate with differences in the body size
distributions, metabolic modes and the trophic positions of taxa in each
system. For our purposes it was necessary that each food web be complete
(i.e., not confined to a guild or interaction module) and only 69 webs
in the database met this criterion. In addition to these webs, we added
several large, recently published food webs, for a total of 77 food webs
in the analysis (23 freshwater, 34 marine, and 20 terrestrial systems),
ranging in web size from 8-224 taxa. The entire list and relevant
descriptors of each web have been made available on-line
(http://waynerossiter.com/ Research/). One web, Jennings Environmental
Center, was produced by us for this paper and has also been made
available on-line (http://waynerossiter.com/Research/) with the
searchable web matrix available through GlobalWeb
(http://globalweb.com/). Because humans and parasites were consumer
types that were not represented in all webs (and would significantly
bias our metrics for food webs for which they're reported), they
were removed entirely from the study. All food webs either already
existed as, or were converted to, binary consumer-based matrices (see
Dunne, 2006, for a formal description of this type of matrix).

We predicted the relative proportion of endothermic taxa in a food
web would be negatively correlated with mean food chain length (MFCL).
However, the relative species richness of endotherms is only a coarse
metric in this regard. We felt that it was also necessary to consider
the relative number of endothermic and ectothermic trophic interactions
in each web (i.e., not only to determine how many endothermic taxa are
present, but assess their relative contribution to each food-web
network). To examine the relationship between the trophic positions of
endothermic taxa and the resulting food chain lengths, food webs were
deconstructed into collections of unique food chain paths (FCPs). That
is, we evaluated all "realized" paths from resources to
terminal predators in each food web (here we mean only FCPs derived from
each food-web interaction matrix, and not simply all possible FCPs).
Trophic position (or height) is typically defined as the one plus the
average trophic position of the species' prey or resources. Any
heterotrophic species (taxon) could potentially appear in multiple food
chains and at different trophic levels. However, none of our analyses
sought to assess directly the average trophic position of individual
species (taxa) but rather the relative proportion of endotherms at each
trophic level across all FCPs in a system. We excluded circularities
("loops") and cannibalism from these calculations. This
permitted us to assess the relative contributions of endotherms and
ectotherms to the total number of trophic interactions (both within
trophic levels and globally), as well as whether or not paths containing
more endotherms, or endotherms at particular trophic positions, produced
shorter food chain lengths in each web.

We also sought to consider the role that body size plays in
determining food chain lengths and perhaps even food-web structure.
Therefore, we also estimated the average mass of each species in each
web, where such distinctions were possible (some low-resolution
groupings did not allow for the approximation of masses). For some webs
the precise masses of species have been previously published (e.g.,
Tuesday Lake, Carpinteria marsh, Raritan river, Muskingum brook, etc.),
and they were included here. Where study-specific values were not
available, values from existing literature were used. For example
FishBase.org was utilized for estimating the average masses of most
fishes. Because most food webs either do not separate juveniles and
adults (i.e., they are represented as one node or unit), or they do not
consider the diets of juveniles at all, we used average masses for
adults. Where juveniles were treated separately, we also did so. The
average masses of many other vertebrate species were also easily
retrievable from the literature. This enabled us to closely approximate
the masses of other similar taxa where no records were available. As in
Yodzis (1984), our goal was to compare masses between and across taxa at
the level of orders of magnitude (i.e., at the log-scale), making any
small errors in our estimates of little consequence. We were able to
approximate masses for taxa in 72 of the food webs in our dataset. This
information was used to supplement each food-web matrix, allowing us to
evaluate the effect of body size along with metabolic mode, trophic
position, and the relative frequencies of trophic interactions.

STATISTICAL ANALYSIS

For each food web, we examined numerous variables related to food
web network structure, metabolic mode, and body size. The following
variables were considered: system type (freshwater, marine, or
terrestrial), species richness, the number of food chain paths (FCPs),
mean food chain length (MFCL), the relative proportion of endotherms at
the herbivore level, maximum food chain length (FCL), connectance,
linkage density, average diet breadth, average prey vulnerability,
average body size at the autotroph, herbivore and apex predator
positions, and the difference between predator-prey body size at the
herbivore and apex predator positions. Because both MFCL and maximum FCL
varied across food webs, it was not possible to compare the proportion
of endotherms or the average body sizes of omnivorous and mid-level
predators. For example the apex predators in one food web might occupy
the fourth trophic level, while those of another food web might occupy
the sixth. However, it was possible to look at these attributes in the
"apex" predator levels of our webs (regardless of respective
trophic level).

As a first pass through the data, we assessed pairwise correlations
between all variables using a restricted maximum likelihood (REML)
multivariate method offered in JMP 11.0. This method produces unbiased
estimates of variance and covariance. We were specifically interested in
determining which variables were highly correlated with MFCL, and the
degree of covariation among those variables. We also utilized multiple
linear regression (MLR) in establishing which variables might serve as
good predictor variables for MFCL and which could be excluded. Variables
related to system size were most strongly correlated with MFCL. However,
several variables necessarily covaried (for example species richness in
combination with linkage density directly determines the number of FCPs
in any web). Additionally, species richness (as well as the number of
FCPs) is known to be heavily biased by taxonomic resolution, which is
not a biologically real property of any system. To control for this
effect, we used best-fit models for each system type to transform MFCL
values so that they were flat with respect to (i.e. not related to)
species richness. We then used pairwise Kolmogrov-Smirnov tests to
evaluate the role of system type and the number of FCPs in predicting
MFCL.

Because of the linked covariation among species richness and the
number of FCPs, and the potential sampling bias observed in species
richness, we removed theses variables from further analyses. We also
excluded variables related to the body size and metabolic mode of apex
predators, because they were not significantly correlated with MFCL.
Using MFCL as the response variable, we then constructed generalized
linear models (GLMs) for both the entire data set (all systems combined)
and individual system types (marine, freshwater and terrestrial). In
general variable selection (from the remaining variables) for our models
was determined in a "backwards" fashion (James et al. 2013),
where variables producing the largest P-value were excluded in a
sequential way. Based on the optimal variable set (as established by
corrected AIC scores as described in Akaike, 1974) for the entire data
set, we applied the same variable set to the GLM for each system type.
Prior analysis indicated that the distributions of our variables were
within the bounds in which the assumptions of a Normal distribution
model using the Identity link function were appropriate. Goodness-of-fit
was defined as [[summation].sub.i] [w.sub.i][([y.sub.i] -
[[mu].sub.i]).sup.2] and statistical significance was evaluated by
Pearson's chi-squared statistic.

Finally, we assessed the relative strength and importance of
predictor variables using Classification and Regression Tree (CART)
models (again excluding species richness and the number of FCPs) to
establish a hierarchical series of optimal "splits" in the
data based on optimized cut-points for predictor variables. Here, the
model seeks to find optimal cut-points for partitioning data based on a
response variable (in our case, MFCL). The predictor variable that best
allows the partitioning of the data based on MFCL would represent the
first cut-point. One of the strengths of this type of analysis is that
it allows for the discovery of cut-points along the distribution of one
predictor variable. For example the resulting cut-point will not simply
be some variable (say, the body size of herbivores) but will be
represented as the point along that distribution allowing for the best
partitioning of the data (a partition above and below a specific body
size). That is, CART analysis allows us not only to determine which
variables are the best predictors of MFCL (which our other analyses
already do) but also a more refined determination of the range (or
thresholds) for which those variables are most important. However, it is
important to mention that CART models are known to over-fit data (Gray
and Fan, 2008), particularly when multiple hierarchical splits are
requested (i.e., the model is capable of continuing to subdivide a
single variable repeatedly). We were only interested in the first and
second optimal splits, after which predictive power of subsequent
variables typically declines precipitously (based on LogWorth values),
and therefore overfitting by repeated partitioning of a single variable
was not a concern. Because we anticipated marked differences among
system types, the same procedures were applied to system-specific
partitions of the data (marine, freshwater, terrestrial), in order to
examine differences in the predictive power of variables in each system
type. These models were also constructed using the statistical software
package JMP 11.0.

RESULTS

Overall, mean food chain length (MFCL) ranged from 2.88 to 6.87
across food webs. The largest MFCL for any terrestrial system was 5.40,
while nearly a quarter (24.6%) of the MFCLs in aquatic food webs
(freshwater and marine) exceeded this value. MFCL increased linearly
with the logarithm of species richness but on different slopes for
aquatic and terrestrial systems (t-value 6.38, P < 0.0001; Fig 1A).
After controlling for species richness, MFCL did not vary significantly
between marine and freshwater systems (Kolmogrov-Smirnov test, D = 0.15,
P = 0.93) but did when comparing aquatic and terrestrial systems (D =
0.45 and 0.40, P = 0.01 and 0.06 for marine-terrestrial and
freshwater-terrestrial pairwise comparisons respectively). That is,
system type (aquatic vs. terrestrial) strongly influenced MFCL.

In our assessment of correlations using REML, the number of food
chain paths (FCPs) was strongly correlated with MFCL in all system types
(statistically significant at P < 0.0001 in all cases). Notably,
terrestrial systems added fewer FCPs in relation to species richness
(Fig. 1B) than did aquatic systems, connectance values for terrestrial
systems were significantly lower than those of aquatic systems (D =
0.49, P = 0.001), and connectance was negatively correlated with species
richness (Correlation = -0.41, P = 0.0005). However, there was no
statistical difference between the total number of links (trophic
interactions) among aquatic and terrestrial systems with respect to
species richness (data not shown).

In considering variables related to the energy efficiency
hypothesis, terrestrial food webs contained many more endothermic taxa
than their aquatic counterparts. On average, 35.8% of the species making
up terrestrial webs were endotherms (SE = 4.57), while endotherms
comprised just 13.84% (SE = 2.81) of marine webs and only 3.85% (SE =
1.43) of freshwater webs (but see discussion). In terrestrial systems
endothermic taxa also occurred more frequently at lower trophic levels.
Eighty-five percent of terrestrial webs contained endotherms at the
herbivore level (compared to 12.1% and 4.3% in marine and freshwater
food webs respectively). In contrast endotherms were scarce in the lower
trophic levels of marine and freshwater food webs, but were more common
atop food chains (particularly food chain paths that were above the MFCL
in each system; Fig. 2). Within terrestrial systems endotherms displayed
larger trophic breadths and smaller vulnerability values than ectotherms
on the same trophic level (Fig. 3). Body mass distributions varied
dramatically within and between trophic levels and across system types.
Specifically, the average masses of autotrophic and herbivorous taxa in
marine and freshwater systems were significantly smaller than those of
terrestrial systems (Fig. 4A). This also resulted in significant
differences in the average masses of trophically linked herbivore and
autotroph taxa for respective systems (Fig. 4B).

After removing species richness and FCPs, our multivariate analyses
indicate the average (log) body mass of herbivorous taxa and, in aquatic
systems, the body mass of their predators, best predicted MFCL.
System-specific GLMs indicated that the average difference in body
masses between linked species in trophic levels two and three was the
best predictor of MCFL in aquatic systems ([X.sup.2] = 10.78, P = 0.001
and [X.sup.2] = 14.18, P = 0.0002 for freshwater and marine systems
respectively). However, in our GLM for terrestrial systems, the average
mass of herbivorous taxa (trophic level two) was decidedly the best
predictor of MFCL ([X.sup.2] = 20.60, P < 0.0001). This finding was
largely corroborated by our CART analysis, which determined the average
mass of herbivorous taxa to be the strongest predictor variable of MFCL
in aquatic systems (SS = 11.47, LogWorth = 3.85, cut point at 1.92 and
SS = 21.08, LogWorth = 5.49, cut point at 0.55 for freshwater and marine
systems respectively). In contrast CART analysis for terrestrial systems
found that the relative proportion of endothermic taxa at the herbivore
level was the most powerful predictor of MFCL (SS = 3.07, LogWorth =
2.02, cut point at 0.15).

DISCUSSION

There remains great interest in the factors that determine food
chain lengths in natural communities, and a single comprehensive theory
remains elusive (Post, 2002; Shurin et al., 2006). In the present study,
we used a large dataset of published food webs to examine the
relationships between food chain length and variables related to body
size and metabolic mode--in the context of food-web structure. Like
previous studies (e.g., Yodzis, 1984; Hairston and Hairston, 1993;
Shurin et al., 2006), we observed marked differences in the mean food
chain lengths (MFCLs) of aquatic and terrestrial food webs. These
differences were separable from species richness but were highly
correlated with the total number of food chain paths (FCPs). Aquatic
food webs displayed more FCPs than terrestrial food webs of equal size
(both in terms of species richness and in the total number of trophic
links), suggesting a difference in the trophic structuring (connectance
values were also lower in terrestrial systems). This might also explain
why maximum food chain lengths varied with system type but not with
species richness. In aquatic (namely marine) systems, while rare,
particularly long food chain paths were possible (as long as 12 links),
while the longest terrestrial food chains path was seven links. (It must
be noted that these maximum lengths are much larger than the MFCL values
for each system, and are rare trophic paths).

One common explanation for the observed differences in MFCL between
aquatic and terrestrial systems is the severe constraint on the range of
body sizes displayed in terrestrial systems (Cohen et al., 1993;
Jennings and Warr, 2003). In aquatic systems food chains are often
filled with small prey items (pico-, nano-, microplankton, meiophauna,
etc.), and many top predators are much larger than those of terrestrial
systems. According to classic cascade models of food webs (i.e.,
Roberts, 2003), body size and gape limitation organize trophic
interactions. For example Brose et al. (2006) assessed 16,863
consumer-resource links spanning ten high-resolution interaction
networks (most of which were trophic guilds, as opposed to complete food
webs) and found that the differences between the masses of linked
consumer and resource taxa in aquatic systems are significantly larger
than those of terrestrial systems. Presumably, a wider the range of body
sizes moving from autotrophs to top predators in a system permits longer
food chains. Our findings support the hypothesis that body size
distribution constrains food chain lengths. Aquatic webs contained, on
average, significantly smaller autotroph and herbivore taxa and (in the
case of marine systems) larger top predators. The importance of this
pattern was detected by our multivariate analyses, which singled out the
average (log) body mass of herbivores (and in the case of aquatic
systems, their predators) as strong predictors of MFCL. That is, the
presence of large herbivorous species was strongly associated with
shortened MFCLs in systems (regardless of ecosystem type).

Most consumer-resource interactions among small organisms are
complete consumption events (as opposed to grazing), and it may be that
biomass flux to higher trophic levels is much greater in systems with
smaller organisms at their bases. Body size is also strongly
(negatively) correlated with generation time and abundance (e.g., Cyr et
al., 1997), meaning total biomass production (and flux) for small
organisms may be large relative to bigger taxa (Benke and Huryn, 2010),
and the microbial loops of aquatic systems are known to be larger than
those of terrestrial systems. Therefore, we cannot discount the
possibility that body size might still fall within the explanatory
umbrella of the nutrient limitation hypothesis (Arim et al., 2007).
Additionally, it must be noted that, while bacteria (and other
components of the microbial community) are present in terrestrial
systems, they are poorly resolved in (and often completely absent from)
published terrestrial food webs.

While body size was detected as a major driver in MFCL for all
systems, our analyses also indicated metabolic mode in herbivores was
important in determining the MFCLs of terrestrial systems. Broadly,
endothermy was much more common in terrestrial systems, and endotherms
assumed lower trophic positions (namely, herbivory), both of which were
negatively correlated with MFCL. More specifically, the presence of
endothermic taxa at the herbivore level was determined as the strongest
predictor variable of MFLC in terrestrial systems, as opposed to body
size in aquatic systems. While the reporting of larger endotherms in
terrestrial food chains may be more resolved than for their smaller
ectothermic counterparts, it seems evident that there are more
endothermic herbivores in terrestrial systems than aquatic ones. In
terrestrial systems macroflora are also usually well described and may
be over-represented in our food webs. However, any bias likely has
little relation to our overall findings, as the average size differences
between aquatic and terrestrial autotrophs was not a significant
predictor of MFCL.

Terrestrial systems act as a test for the role of endothermy in
limiting food chain lengths, as endotherm and ectotherm consumers are
often more similar in size in those systems (partially controlling for
body size as a variable). Our CART analysis suggested that endothermy
(and not larger body size) was the best predictor of MFCL in terrestrial
systems. These findings are consistent with Ryszkowski and French
(1982), who directly implicated the high energetic cost of endothermy
and the reciprocal low conversion efficiency with respect to biomass
production as important drivers in the biomass patterns and community
structuring of terrestrial ecosystems (though they did not test the
hypothesis). This is also consistent with the more recent work of Shurin
and Seabloom (2005), in which ectothermic species were found to better
propagate trophic cascades than endotherms, as a direct consequence of
their increased metabolic efficiency. However, those authors also
implicated size differences between herbivores and autotrophs as
important predictors of the strength of trophic cascades (see also
DeLong et al. 2015).

Given that body size was implicated as the most important driver in
aquatic systems (which have larger body size spectra), but not in
terrestrial systems, we believe that endothermy may be hierarchically
nested within the more overarching role of body size in determining food
chain length. It may be that, within the narrow range of body sizes seen
in terrestrial systems, endothermy becomes a much more relevant factor
in constraining trophic interactions (and energy flux). It is
interesting to note that two of the largest MFCL values for terrestrial
systems came from food webs that reported only ectotherms (WEB151 and
WEB311, additional information on these WEBs are available on-line at
http://waynerossiter.com/Research/), even though WEB151 contained just
23 taxa. Here, the size distribution was considerably constrained. This
pattern was not seen in aquatic systems, and supports the idea that
endothermy plays a more pivotal role in determining food chain lengths
within a narrowed range of consumer-resource body sizes.

Taken together, the body size, relative fraction and trophic
position of endotherms in aquatic and terrestrial systems may play into
the previously discussed differences in FCPs. Because metabolic
requirements are roughly an order of magnitude greater for endotherms
than ectotherms of the same size, it may be that endothermic taxa
bottleneck energy, limiting the total number of trophic interactions
(and chains) for entire food webs. This view is supported by the
observation that endotherms have fewer predators (limiting the number of
FCPs up food chains) and depend upon more prey items (which might be
linked to increased energetic requirements). Our findings are consistent
with Yodzis (1984), who demonstrated that ectotherms are more likely to
support consumers than endotherms, as well as the data used in Cohen et
al. (1993), which contained far more ectothermic trophic interactions
than endothermic ones, even when considering only trophic links to
vertebrate predators. Therefore, we suggest endothermy may limit food
chain lengths by altering food-web structure and complexity. This
however does not necessarily explain why many marine food webs with long
MFCLs contain endotherms as top predators, and it is worth noting Vander
Zanden and Fetzer (2007) found marine food chains containing mammals
were longer than those without them. However, we point to the facts
these mammals are typically top predators, rather than consumers at the
herbivore and omnivore levels (as is seen in terrestrial systems), and
that they are likely fewer in number and smaller in population biomass.
Still, these findings need further reconciliation going forward.

Acknowledgments.--We thank Jennifer Dunne and Ross Thompson for
providing access to the GlobalWebs database. We also thank Kenneth
Elgersma and Givonni Strona for advice and discussion regarding
analysis. Finally, we thank Brandi L. Miller-Parrish and the PADEP for
providing us with access to the survey studies and data for Jennings
Environmental Education Center (Slippery Rock, PA, U.S.A.).

Caption: Fig. 1.--(A) Distribution of MFCLs against species
richness (log-transformed) for aquatic and terrestrial ecosystems.
Aquatic systems demonstrate significantly longer MFCLs as species
richness increases (see results for statistical tests). (B)
Relationships between MFCL and the number of FCPs (log-transformed) in
each system type. There was no statistical difference between the slopes
for freshwater and marine systems, but terrestrial systems produce
significantly smaller MFCLs in relation to increasing FCPs

Caption: Fig. 2.--Average relative proportions of taxa that are
endothermic at each trophic position in food chains for respective
systems. Endothermy is significantly more prevalent in the lower trophic
levels of terrestrial systems, and declined precipitously beyond trophic
level three. Aquatic systems contained very few trophic interactions
involving endothermic herbivores, but the proportion of endothermic
predators increased with trophic level. In all systems, food chains
containing more than seven links were rare, but consistently contained
endotherms as apex predators in aquatic systems

Caption: Fig. 3.--Mean differences in vulnerability (the number of
predators feeding on a prey species) and trophic breadth (the number of
prey species a predator feeds on) for ectothermic and endothermic taxa
in terrestrial systems. Positive values for vulnerability indicate that,
on average, ectothermic species had more predators than did endotherms
in the same trophic level. Conversely, negative trophic breadth values
indicate that endothermic predators fed on more prey species than did
ectotherms in the same trophic level. Differences were calculated for
each food web and were averaged across all terrestrial systems. Error
bars indicate SE

Caption: Fig. 4.--(A) Average body size (mass) of autotrophs and
herbivores (trophic levels one and two), and top predators (T2 and T1)
in each system. (B) Average difference in the body masses of resource
and consumer species at the bottom two and top two trophic levels for
each system type. Because most terrestrial food webs contain five or
less trophic levels, those top predators (trophic level five) are
compared to their aquatic counterparts, which may sit at much higher
trophic levels, but still represent top predators

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